Inquiring Minds

Questions About Physics

Question about Radioactive Decay

Dear Webmaster,

What is the actual 'event' that causes an unstable isotope (Ex: C(14)) to decay? I know that a neutron splits into an electron and a proton during radioactive decay, but what causes this to happen? Is it a roll of the dice, or some nuclear force gone astray?

Thanks for your help!

Dave Morris

Dear Dave,

The answer to both your questions is YES ! Sort of. The decay is random, but the probability of it taking place depends on the nuclear force (but not on the force going astray in any way, though). It depends on the force acting like it is supposed to.

The explanation is somewhat of an exposition on how quantum mechanics works. This is a great question.

Take a simpler example like a hydrogen atom. The atom is composed of a proton and an electron bound together because both have a electric charge and the signs of the charges are opposite so they attract. The atom can exist in various bound states. Each has an energy level. You could also describe the state as saying the average distance between the proton and the electron revolving around it is so and so. This quantity and the energy of the state will have some relation. What makes one state want to decay down to another giving off a photon of light in the process? After all, this is how we get fluorescent light, for example.

Well, the process of decaying from one state to another (of lower energy) is a random process, BUT the overall probability of it taking place at certain time is a known function of time. In other words I can say it has a 35% probability of decaying during the next second. That doesn't mean it necessarily will.

What determines that probability is the nature of the electromagnetic force acting (attracting) between the two particles (proton and electron). In other words, the static properties of the system (energy levels) are determined by the nature of the force as well as the dynamic properties such as the probability of decaying between two levels. It is not the force gone astray though. It is the force acting as it is supposed to.

I apologize since part of this understanding relies on quantum mechanics. You may be thinking that such a process needs to have a precisely determined cause and effect at a certain time and place. Quantum mechanics says this is not so. You can predict what you get if you wait sufficiently long enough but not when you will see a specific event. You can only get the probability of it happening at that time.

So is the same for your neutron. The neutron will decide to decay at some random time and you don't exactly know when, but how often it does so (i.e. the probability of it happening again within the next minute, given you just observed one) is known by understanding the structure of the nuclear forces that hold the neutron together (specifically, the weak nuclear force).

In more particle physics terms, the neutron consists of 3 quarks (up, down, down). The proton consists of 3 quarks (up,up,down). There is some probability one of the neutron's down quarks will want to change into an up quark by giving it a W boson, which is one of the force carriers of the weak nuclear force. Again the probability for this to take place is governed by how strongly the down quark wants to "couple" to the W boson, (basically its weak nuclear "charge"), and some properties of the W boson itself. So you can measure how often it takes place, it just doesn't do so at a nice heartbeat sort of rhythm, it is more choppy, but again, if you wait long enough, you can measure the overall rate.

It is not from some transient in the structure of the force though, I want to emphasize.

It is actually a great thing to ponder on in order to learn quantum mechanics which is not an easy subject to get a real good understanding. There are plenty of us who learn the equations quickly but it takes quite a bit longer to really understand how Q.M. operates. I hope this helps.